This could be reasonably expressed. Flatpak can sort of do this via portals, you have a filepicker that can see the host system and the picked file is transparently mounted into the app container.
For multiple files per document, the solution would be either multi-file select (bad) or allowing apps to specify very simple rules for additional files, ie "removing extension, also mount all files starting with that name".
The filepicker could integrate that and either show all those files as a single document or select all files for a document visible, so the user is informed.
For multiple files per document, the solution would be either multi-file select (bad) or allowing apps to specify very simple rules for additional files, ie "removing extension, also mount all files starting with that name".
The filepicker could integrate that and either show all those files as a single document or select all files for a document visible, so the user is informed.